A drive shaft, constant velocity joint outer race, constant velocity joint inner race, and other shaft parts forming the power transmission system of an automobile usually are produced by shaping medium carbon steel into predetermined part shapes by cutting, rolling, forging, etc. and then induction hardening and tempering them.
“Induction hardening” is the method of placing a part to be heated inside of a heating coil which is connected to a high frequency power supply, running high frequency current through the coil to generate an alternating magnetic flux, causing eddy current loss and hysteresis loss at the part to generate heat, and thereby hardening the part. Usually, it is performed for the purpose of hardening the surface.
In heating by a high frequency, in principle, step parts or groove parts of the parts are more easily insufficiently raised in temperature compared with other locations. Projecting parts are more easily overheated compared with other locations. Therefore, the temperature will not become uniform in the part as a whole.
For this reason, for induction hardening, heating is necessary so that the location as a whole treated by induction hardening is not insufficiently raised in temperature.
However, if heating so that the location as a whole treated by induction hardening is not insufficiently raised in temperature, a projecting part will become overheated in state by as much as 1100 to 1200° C. or so. For this reason, the austenite crystal grains will coarsen and accordingly the part will be distorted. Here, “coarsening” means specifically becoming a crystal grain size of less than no. 6.
As a measure against this, at the present, the practice is to use a special heating coil for each part and to strictly adjust the distance from the part or pretreat the part for reducing the temperature difference to keep the projecting part from becoming overheated in state as much as possible. However, these measures are not necessarily perfect.
PLT 1 discloses induction hardened steel, which prevents grain coarsening, which contains, in addition to predetermined amounts of C, Si, etc., by mass %, Ti: 0.05 to 0.20% and N: less than 0.01% and causes Ti carbides and Ti carbonitrides to finely disperse into the steel.
PLT 2 discloses a part for induction hardening use which contains, in addition to predetermined amounts of C, Si, etc., by mass %, Mo: 0.05 to 2.0% and which defines the working conditions of the material so as to cause Mo precipitates to become finer and make the crystal grains finer.
However, even if using the steel or working conditions which are disclose in PLT 1 and PLT 2, when locally becoming an over 1100° C. overheated state such as when induction hardening a part having a projecting part, coarsening of the austenite crystal grains at that location could not be prevented.